The ancients thought that stars were fixed pinpoints of light on the sky. Today we know that they are all moving, like fish in a pond. This so-called proper motion is so small that it is not noticeable to the human eye over a single lifetime. But Hubble can precisely track stellar motions to razor-sharp precision. Not surprisingly the nearest star to our Sun, Proxima Centauri, is one of the fastest moving across the sky. Hubble astronomers have found that it will pass in front of two far-more distant background stars, once in 2014 and again in 2016. This will afford a very rare opportunity to see how Proxima's gravity warps the image of the background stars by bending their light. This effect, called gravitational lensing, can be used to estimate Proxima Centauri's mass and establish the presence of any planets orbiting the star.

_________________The universe is not only stranger than we imagine, it is stranger than we can imagine.

We have found that Proxima Centauri, the star closest to our Sun, will pass close to a pair of faint background stars in the next few years. Using Hubble Space Telescope (HST) images obtained in 2012 October, we determine that the passage close to a mag 20 star will occur in 2014 October (impact parameter 1.6"), and to a mag 19.5 star in 2016 February (impact parameter 0.5"). As Proxima passes in front of these stars, the relativistic deflection of light will cause shifts in the positions of the background stars by ~0.5 and 1.5 mas, respectively, readily detectable by HST imaging, and possibly by Gaia and ground-based facilities such as VLT. Measurement of these astrometric shifts offers a unique and direct method to measure the mass of Proxima. Moreover, if Proxima has a planetary system, the planets may be detectable through their additional microlensing signals, although the probability of such detections is small. With astrometric accuracies of 0.03 mas (achievable with HST spatial scanning), centroid shifts caused by Jovian planets are detectable at separations of up to 2.0" (corresponding to 2.6 AU at the distance of Proxima), and centroid shifts by Earth-mass planets are detectable within a small band of 8 mas (corresponding to 0.01 AU) around the source trajectories. Jovian planets within a band of about 28 mas (corresponding to 0.036 AU) around the source trajectories would produce a brightening of the source by >0.01 mag and could hence be detectable. Estimated timescales of the astrometric and photometric microlensing events due to a planet range from a few hours to a few days, and both methods would provide direct measurements of the planetary mass.

Assuming that the centroid shifts of the two sources can be measured with an accuracy of 200 mas per epoch, Proxima’s mass can be measured with an accuracy of ∼5%.

Hey, that's great! 5% of 0.12 M☉ is 0.006 M☉, or about 6 MJ. That's a very healthy accuracy for such a late-type star, And would be an excellent addition to the M-dwarf mass-luminosity relation.

If Proxima has planets in an "expectable" system (something like Kepler-42=KOI-961), then it's unfortunately rather likely that these events won't turn up anything. Still, there is the off-chance that an Earth-mass or Super-Earth-mass planet will be close enough to the background star to be detectable - which would be more than enough to make the search worthwhile!

PlutonianEmpire wrote:If Proxima desides to flare during one of the events, will that cause any issues or make observations more difficult?

I'm going to go with probably not. What will actually enable the detection of a companion is the presence of the background star, so Proxima's exact character isn't so important. The lensing events should also be considerably longer than any single flare event, so it likely wouldn't be that major anyway.

Assuming that the centroid shifts of the two sources can be measured with an accuracy of 200 mas per epoch, Proxima’s mass can be measured with an accuracy of ∼5%.

Hey, that's great! 5% of 0.12 M☉ is 0.006 M☉, or about 6 MJ. That's a very healthy accuracy for such a late-type star, And would be an excellent addition to the M-dwarf mass-luminosity relation.

Yes, personally I think the opportunity for a direct mass measurement of a well-observed low-mass star is the most interesting thing about this. Any planets are a bonus, but I'm not getting my hopes up.

The paper gives the date of closest approach for source 1 as October 20th, uncertainty ± 10 days. Will probably be a while before we hear anything (actually come to think of it, do we know if the observation proposal was accepted?).

There's an update on this in the comments of the Centauri Dreams blog. Observations of the 2015 event were not sufficient to search for planets (though should be possible to get the mass measurement), hopefully the 2016 event will be followed with a higher frequency.